Electrical impedance tomography (EIT) is an imaging technique in which an image of the impedance distribution of a part of the body is inferred from surface electrical measurements. EIT is a non-invasive method in which an alternating current of a few mA with a frequency of for example 50 kHz is fed into the human body and the resulting surface potentials are measured at different points around the body, using either conventional ECG electrodes or customized electrode belts. Two-dimensional distribution images or tomograms can be generated, which represent the distribution of the bioelectrical properties in one cross-sectional plane defined by the position of the electrodes around the thorax, by the successive rotation of the current feed points and the simultaneous measurements of the resulting surface potentials and by applying mathematical reconstruction algorithms to obtain the images. Such tomograms of the distribution of bioelectrical properties are of special interest in medicine in particular for determining functional lung characteristics, because the electrical impedance of lung tissue changes with the air content. It has been shown that ventilation induced impedance changes and volume changes show a high correlation with data determined with computer tomography (T. Meier at al., Intensive Care Med. (2008) 34: 543-550).
The basic principle for recording impedance distribution data using an electrical impedance tomography system is for example described in WO 91/19454 in which also the basic principles of the projection algorithms for deriving the image data are described.
There have also already been approaches how EIT could be used to monitor the effects of ventilation therapy and to control the ventilation therapy, as for example in DE 10 2006 018 198 A1 and DE 10 2006 018 199 A1. In these publications the underlying general approach is to always compare two different states with each other and to control a connected ventilator (also known as a respirator) based on the determined differences.
In EP 1 137 365 A1 the general approach described is to use a ventilator to apply two different pressures and to use EIT to analyze the impedance changes induced by this pressure change to determine the opening pressure of lung zones.
In EP 1 292 224 A1 a method for displaying information obtained by electrical impedance tomography data is described. In particular it is described that various screen modes of an EIT monitor are automatically adjusted based on pathological conditions. In this respect a phase lag mode is specifically described which analyzes the time delay (phase lag) of regional end-inspiratory or end-expiratory peaks. While the phase lag mode analyzes the delay of the regional beginning of inspiration in a complete breath it is not sensitive to changes occurring within the inspiratory and expiratory phase, therefore does not allow a detailed analysis of functional lung characteristics.
Another known measuring system for electrical impedance tomography is described in EP 1 000 580 A1 in which the graphic display of the measured impedance values is superimposed by the display for the same body slice in order to allow a more accurate evaluation of the measurements performed by means of electrical impedance tomography.
Functional lung characteristics which have been identified as particularly useful in connection with the present invention are the contributions of particular areas of the lung to the total tidal breathing activity at various times over the breathing cycle, or in other words the intratidal gas distribution over the lung. The determination of such functional lung characteristics would be useful to provide adapted settings for the individual patient of artificial respiration equipment. It has been found that from such characteristics setting parameters for artificial respiratory systems may be derived, such as the positive end-expiratory pressure (PEEP), the tidal volume (VT), the expiration time (Te) and the susceptibility of the lung to a so-called recruitment maneuver.
Selecting optimal PEEP and appropriate tidal volume is a difficult task. Widely accepted practice is to use a tidal volume of 6 ml/kg predicted body weight (ARDSnet N. Engl J Med 2000; 342:1301) in spite of the fact that such a tidal volume may well be too large for a “baby lung” of an ALI/ARDS patient, but is at least better than the tidal volumes of 9-12 ml/kg that have been used previously. Selection of optimal PEEP is a real challenge with no satisfying practical answer today.
Originally PEEP was set to optimize oxygenation or oxygen delivery (Suter et al. N Engl J Med 1975; 292:284) but today PEEP is set to mechanically stabilize the lung, i.e. to avoid cyclic opening and collapse. The golden standard, performing a static PV curve and setting PEEP above the lower inflection point and plateau pressure below the upper inflection point at the straight part of the PV curve is not done at the bedside, except in research environments. It is also important to realize that during the straight part of the PV curve recruitment and overstretching of lung regions may occur simultaneously (Hickling. Curr opin, Crit Care 2002; 8:32). Therefore, with the use of these settings, there is no guarantee that cyclic opening and closing are not accompanied by overstretching as well.
A complicated way to set PEEP has been suggested by using CT images, but obviously this is a very “unclinical” method that cannot be used routinely. In the absence of practical clinical methods PEEP is normally set by a combination of an approximation of lung mechanics and oxygenation criteria. Recently, it has been shown that PEEP could be set by monitoring dynamic compliance (tidal volume/end-inspiratory pause pressure-end-expiratory pressure) (Suarez-Sipmann et al. Crit Care Med 2007; 35:214), but this has so far only been done in a surfactant deficient pig model, and furthermore dynamic compliance is a fairly insensitive indicator to properly set PEEP in ALI/ARDS patients. This method relies on opening the lung completely before the PEEP step down procedure. The peak pressure (60 cmH2O) applied to the lung is quite high and could be dangerous as it involves the risk of developing pneumothorax in patients.
Another method for optimization of PEEP and tidal volume is described in EP 1 108 391 A1 which proposes to determine a so-called pulmonary stress index from the pressure time relationship. However, as recruitment and overstretching may occur simultaneously, these conditions might remain undetected due to the fact that these concurrent effects are masked in the global pressure curve, which only reflects the overall behavior and resulting effects of the inhomogeneous lung as a whole.